We’ve all seen margin: 0 auto; for horizontal centering, but margin: auto;has refused to work for vertical centering… until now! But actually (spoiler alert!). I’m not the pioneer of this method, and it may even be a common technique, however, most vertical centering articles never mention it and I had never seen it until I dug through the comments section of a particular article. There, Simon linked to this jsFiddle that blew every other method out of the water (the same method was also mentioned by Priit in the comments). Researching further, I had to use very specific keywords to find some other sources for this method.

BROWSER COMPATIBILITY:

Absolute Centering was tested and works flawlessly in the latest versions of Chrome, Firefox, Safari, Mobile Safari, and even IE8-10.

Explanation

After researching specs and documentation, this is my understanding of how Absolute Centering works:

In the normal content flow, margin: auto; equals ’0′ for the top and bottom.W3.org: If ‘margin-top’, or ‘margin-bottom’ are ‘auto’, their used value is 0.

position: absolute; breaks the block out of the typical content flow, rendering the rest of the content as if that block weren’t there.Developer.mozilla.org: …an element that is positioned absolutely is taken out of the flow and thus takes up no space

Setting top: 0; left: 0; bottom: 0; right: 0; gives the browser a new bounding box for the block. At this point the block will fill all available space in its offset parent, which is the body or position: relative; container. Developer.mozilla.org: For absolutely positioned elements, the top, right, bottom, and left properties specify offsets from the edge of the element’s containing block (what the element is positioned relative to).

Giving the block a width or a height prevents the block from taking up all available space and forces the browser to calculate margin: auto based on the new bounding box.Developer.mozilla.org: The margin of the [absolutely positioned] element is then positioned inside these offsets.

Since the block is absolutely positioned and therefore out of the normal flow, the browser gives equal values to margin-top and margin-bottom centering the element in the bounds set earlier.W3.org: If none of the three [top, bottom, height] are ‘auto’: If both ‘margin-top’ and ‘margin-bottom’ are ‘auto’, solve the equation under the extra constraint that the two margins get equal values. AKA: center the block vertically

Absolute Centering appears to be the intended use for margin: auto; based on the spec and should therefore work in every standards compliant browser.

Offsets

If you have a fixed header or need to add other offsets, simply add it in your content block’s styles like top: 70px;. As long as margin: auto; is declared, the content block will be vertically centered within the bounds you declare with topleftbottom and right.

You can also stick your content block to the right or left while keeping it vertically centered, usingright: 0; left: auto; to stick to the right or left: 0; right: auto; to stick to the left.

Overflow

.Absolute-Center.is-Overflow { overflow: auto; }

Content taller than the block or container (viewport or a position: relative container) will overflow and may spill outside the content block and container or even be cut off. Simply addingoverflow: auto will allow the content to scroll within the block as long as the content block itself isn’t taller than its container (perhaps by adding max-height: 100%; if you don’t have any padding on the content block itself).

Resizing

You can resize your content block with other classes or Javascript without having to recalculate the center manually! Adding the resize property will even let your content block be resized by the user.

Absolute Centering keeps the block centered no matter how the block is resized. Setting min-/max- will limit the block’s size to what you want and prevent it from overflowing the window/container.

If you don’t use resize: both, you can add a transition to smoothly animate between sizes. Be sure to set overflow: auto since the block could be resized smaller than the content.

Absolute Centering is the only vertical centering technique tested that successfully supports theresize: both property.

LIMITATIONS:

Set your max-width/max-height to compensate for any padding on the content block itself, otherwise it will overflow its container.

The resize property is not supported on mobile browsers or in IE 8-10 so make sure your users have an alternate way of resizing if that is essential to user experience.

Combining resize and transition properties causes a delay equal to the transition time when the user attempts to resize.

Images

Images work too! Apply the class/style to the image itself and set height: auto; like you would with a responsively-sized image to let it resize with the container.

Note that height: auto; works for images, but causes a regular content block to stretch to fill the container unless you use the variable height technique. It’s likely that because browsers have to calculate the height for the image rendered image, so margin: auto; ends up working as if you’d declared the height in all tested browsers.

Absolute Centering does require a declared height, however the height can be percentage based and controlled by max-height. This makes it ideal for responsive scenarios, just make sure you set an appropriate overflow.

One way around the declared height is adding display: table, centering the content block regardless of content length. This causes issues in a few browsers (IE and Firefox, mainly), so my buddy Kalley at ELL Creative wrote a Modernizr test to check if the browser supports this method of centering. Now you can progressively enhance

LIMITATIONS:

This will break cross-browser compatibility. You may want to consider an alternate technique if the Modernizr test doesn’t meet your needs.

Not compatible with the Resizing technique.

Firefox/IE8: Using display: table aligns the content block to the top, but is still centered horizontally.

IE9/10: Using display: table aligns the content block to the top left.

Mobile Safari: The content block is centered vertically, but becomes slightly off-center horizontally when using percentage based widths.

2. Other Techniques

Absolute Centering is a great solution for centering, but there are other methods that may fit more specific needs. The most commonly used or recommended methods are Negative Margins,Transforms, Table-Cell, Inline-Block, and now Flexbox. They are covered more in depth in other articles, so I’ll only cover the basics here.

2.1 Negative Margins

Perhaps the most common technique. If exact dimensions are known, setting a negative margin equal to half the width/height (plus padding, if not using box-sizing: border-box) along with top: 50%; left: 50%; will center the block within a container.

It should be noted that this is the only method tested that worked as expected in IE6-7.

ADVANTAGES:

Works well cross-browser, including IE6-7

Requires minimal code

LIMITATIONS:

Not responsive. Doesn’t work for percentage based dimensions and can’t set min-/max-

Content can overflow the container

Have to compensate for padding or use box-sizing: border-box

2.2 Transforms

One of the simplest techniques with about the same benefits as Absolute Centering, but supports variable height. Give the content block transform: translate(-50%,-50%) with the required vendor prefixes along with top: 50%; left: 50%; to get it centered.

2.3 Table-Cell

This may be the best technique overall, simply because the height can vary with the content and browser support is great. The main disadvantage is the extra markup, requiring a total of three elements to get the final one centered.

By popular demand: Inline-Block centering. The basic idea is using display: inline-block, vertical-align: middle and a psuedo element to center your content block inside of a container. The concept here is best explained in this CSS-Tricks article, Centering in the Unknown. My implementation has a few new tricks here that I haven’t seen elsewhere that help solve a few issues.

The content block’s width must be declared to be no wider than 100% of the container minus 0.25em if the content is wider than the container. like a block with long paragraph text. Otherwise, the content block will be pushed to the top, which is the reason for using :after. Using :before caused the content to be pushed down 100%!

If your content block needs take up as much available horizontal space as possible, you can add either max-width: 99%;, which works for bigger containers, or max-width: calc(100% - 0.25em)depending on the browsers you support and the width of the container.

The benefits are mostly the same as the Table-Cell technique, but I initially left this method out because it’s very much a hack. Regardless, browser support is great and it proves to be a popular technique.

ADVANTAGES:

Variable height content

Content overflows by stretching the parent element

Works well cross-browser, and can be adapted for IE7 support (view the CSS to see)

LIMITATIONS:

Requires a container

Relies on margin-left: -0.25em; to horizontally center correctly, but may need to be adjusted for different fonts/sizes

Content block’s width must be declared to be no wider than 100% of the container minus 0.25em.

Futher Resources

Read more about Transform Centering in Chris Coyier’s article “Centering In The Unknown” on CSS-Tricks.

The future of layout in CSS, Flexbox is the latest CSS spec designed to solve common layout problems such as vertical centering. Smashing Magazine already has a great article on Centering Elements with Flexbox that you should read to for a more complete overview. Keep in mind that Flexbox is more than just a way to center, it can be used for columns and all sorts of crazy layout problems.

ADVANTAGES:

Content can be any width or height, even overflows gracefully

Can be used for more advanced layout techniques.

LIMITATIONS:

No IE8-9 support

Requires a container or styles on the body

Requires many vendor prefixes with different syntaxes to work on modern browsers

Possible performance issues

Recommendations

Each technique has their advantages. Which one you choose mainly boils down to which browsers you support and what your existing markup looks like. Use any above technique to make the right choice to match the features you need.

Absolute Centering works great as a simple drop-in solution with no-fuss. Anywhere you used Negative Margins before, use Absolute Centering instead. You won’t have to deal with pesky math for the margins or extra markup, and you can size your boxes responsively.

If your site requires variable height content with the best browser compatibility, try out the Table-Cell, Inline-Block techniques. If you’re on the bleeding edge, give Flexbox a try and reap the benefits of its advanced layout techniques.

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